These studies are directed toward determining the molecular lesion in patients with homozygous beta thalassemia. Current evidence indicates that quantitative deficiency of beta globin mRNA might result because of defective transcription ineffective processing, or reduced stability of the final mRNA product. Several avenues of investigation are being pursued. Restriction endonuclease digestion of isolated DNA followed by gel electrophoresis and identification of those fragments containing globin gene sequences has shown that the general structure and organization of the beta globin genes is normal in the 25 patients studied. The DNA from two individual patients has been partially digested with Eco RI, linked to the bacteriophage vector, Charon 4A, and cloned into E. coli. Recombinants containing parts of the beta globin gene have been identified and are under analysis. Impaired stability of beta globin mRNA in one patient seems likely based on equivalent rates of transcription of the alpha and beta globin genes but declining proportions of beta globin mRNA during incubation of erythroid cells in vitro. Finally a technique to study processing has been developed whereby the precursor to mature globin mRNA can be quantitated and compared to the concentration of cytoplasmic beta globin mRNA.